The present invention relates to a fluoride glass optical fiber adopted to effectively transmit light of a wavelength of 2-4 .mu.m.
In the mid-infrared region, it is comparatively easy to obtain a stabilized large output light by use of a gas laser like CO.sub.2 laser, but no medium capable of transmitting the obtained output light with a low loss has yet to be realized. Accordingly, the use of infrared radiation is limited to a few fields such as a surgical scalpel in the medical field, and metal and wooden chip processing field.
It is predicted that some chemical compounds like fluoride, chloride, bromide and iodide allow infrared radiation of 2-4 .mu.m wave length to be transmitted with an extremely small loss of 0.001 dB/km.
Therefore, if an optical fiber like a silica fiber which removes external loss factors completely and displays only its intrinsic loss could be provided, it would then be possible to realize a direct communication system for distances up to 10,000 km in the communication field.
To increase the utilization efficiency of infrared radiation and to enable it to be used in various fields, it is necessary to conceive an optical fiber which allows infrared radiation to pass therethrough with low loss. From this standpoint, several kinds of primitive infrared fibers have been provided and of these fibers, one that has shown a minimum loss consists mainly of fluoride glass but even in this case, the value of the loss is as great as 5-6 dB/km in a 2 .mu.m wavelength range which is far greater than the theoretical value.
The following two factors are considered causes for the inability to achieve a reduction in loss in the fluoride glass optical fiber.
One is the absorption loss due to the existence of water or transition metal ions in the fluoride glass as impurities and the other is the increase in the scattering loss due to the generation of fine crystals in the glass when a preform of the glass is drawn into a fiber by heating.
As described above, in order to make the transmission loss of the fluoride glass fiber approach its theoretical value as close as possible, it is absolutely necessary to reduce both of the above-mentioned adsorption and scattering losses.
The absorption loss is not peculiar to the fluoride glass optical fiber but is common with other optical fibers and it is necessary to avoid it by high purification of the fiber material or the dehydration of glass material as in the case of the silica glass optical fiber.
However, the scattering loss due to the precipitation of fine crystals of the material is not observed in the silica glass optical fiber but is an essential problem resulting from the properties of the fluoride glass itself. To describe the essential conditions required of a glass material for forming an optical fiber, the optical fiber is produced from a block of the material called a preform, by drawing. To draw the glass material, it is heated to a viscosity suitable for drawing. This viscosity is usually 10.sup.5 poise and the temperature at this stage is called a drawing temperature which varies depending on the composition of glass used. Accordingly, to obtain a drawn fiber without generating fine crystals in the glass, it is necessary for the crystallization temperature to be higher than the drawing temperature.
Further, in order for the optical fiber to have a suitable waveguide structure, it is necessary to have a core layer and a clad layer between which a certain refractive index difference exists. This refractive index difference is obtained by varying the composition of the glass material used. Further, the two layers must be drawn simultaneously.
To sum up, the essential conditions required of a glass for an optical fiber are that the glass must include two kinds of composition having refractive indexes different from each other by the required amount, and the crystallization temperatures of both glasses must be higher than the drawing temperature of either glass, whichever the higher.
However, a core-clad composition has not been found that satisfies the above-mentioned conditions.
For example, in the case of the conventional fluoride glass optical fiber belonging to the ZrF.sub.4 -BaF.sub.2 -LiF-LaF.sub.3 -AlF.sub.3 type or the ZrF.sub.4 BaF.sub.2 -NaF-LaF.sub.3 -AlF.sub.3 type, the glass forming the core layer has been added with PbF.sub.2 so as to increase the refractive index of the core layer to be higher than that of the clad layer so that the drawing temperature of the core glass becomes lower than the drawing temperature of the clad. As a result, when the material is drawn into the optical fiber, fine crystals are generated in the core layer causing a high transmission loss.